Differential Diagnosis

The patient's symptoms of fever, lower abdominal pain, and purulent vaginal discharge, along with a tender mass in the right lower quadrant, suggest an infection involving the female reproductive organs. The differential diagnosis can be categorized as follows:

• Single most likely diagnosis

  • Pelvic Inflammatory Disease (PID) with a tubo-ovarian abscess: This is the most likely diagnosis given the patient's symptoms and the presence of a tender mass. PID is a common condition in sexually active young women, and the formation of a tubo-ovarian abscess is a known complication.

• Other Likely diagnoses

  • Ovarian cyst or torsion: Although less likely than PID with an abscess, an ovarian cyst that has become infected or torsioned could present with similar symptoms.
  • Endometriosis: This condition can cause chronic pelvic pain and could potentially lead to the formation of a mass if an endometrioma (a cyst filled with old blood) develops, though it's less likely to cause acute symptoms like fever.
  • Diverticulitis: Although more common in older populations, diverticulitis could present with lower abdominal pain and fever, but the presence of a purulent vaginal discharge and a mass on bimanual examination makes this less likely.

• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)

  • Ectopic pregnancy: Although the patient is sexually active and this could be a concern, the presence of a purulent discharge and the specific location of the pain and mass make this less likely. However, ectopic pregnancy is a medical emergency and must always be considered in women of childbearing age with abdominal pain.
  • Appendicitis: While appendicitis typically presents with pain that migrates to the right lower quadrant, the presence of a purulent vaginal discharge and a mass on bimanual examination makes this diagnosis less likely. However, appendicitis can be life-threatening if not promptly treated.
  • Ovarian tumor: Although rare, an ovarian tumor could present as a mass, and while less likely to cause acute infectious symptoms, it's a critical diagnosis not to miss due to its potential impact on fertility and overall health.

• Rare diagnoses

  • Actinomycosis: A rare bacterial infection that can cause chronic pelvic pain and the formation of a mass, but it's much less common than PID or other conditions listed here.
  • Genital tuberculosis: This could present with chronic pelvic pain, infertility, and masses, but it's relatively rare in many parts of the world and would typically be associated with other systemic symptoms of tuberculosis.